Scaling Theory of Polyelectrolyte Nanogels

The present paper develops the scaling theory of polyelectrolyte nanogels in dilute and semidilute solutions.The dependencies of the nanogel dimension on branching topology, charge fraction, subchain length, segment number,solution concentration are obtained. For a single polyelectrolyte nanogel in salt free solution, the nanogel may be swelled by the Coulombic repulsion(the so-called polyelectrolyte regime) or the osmotic counterion pressure(the so-called osmotic regime). Characteristics and boundaries between different regimes of a single polyelectrolyte nanogel are summarized.In dilute solution, the nanogels in polyelectrolyte regime will distribute orderly with the increase of concentration. While the nanogels in osmotic regime will always distribute randomly. Different concentration dependencies of the size of a nanogel in polyelectrolyte regime and in osmotic regime are also explored.     

Polyelectrolyte condensation induced by linear cations

We examine the role of the condensing agent in the formation of polyelectrolyte bundles, via grand-canonical Monte Carlo simulations. Following recent experiments we use linear, rigid divalent cations of various lengths to induce condensation. Our results clarify and explain the experimental results for short cations. For longer cations we observe novel condensation behavior owing to the alignment of the cations. We also study the role of the polyelectrolyte surface-charge density, and find a nonmonotonic variation in bundle stability. This nonmonotonicity captures two trends that have been observed in separate experiments.     

Water-Soluble Polyelectrolyte Complexes of Oppositely Charged Polysaccharides

The article is an overview of our recent study on some particular aspects of polyelectrolyte complex (PEC) formation by oppositely charged polysaccharides when they are brought into contact in aqueous solutions. This type of complexation can lead to the thickening effect, jellification or PEC precipitation that find numerous applications in a variety of fields from the regulation of rheological characteristics of solutions to fabrication of functional materials by the layer-by-layer technique. Our focus was on the rheological aspects of water-soluble PEC formation and jellification, but to gain an insight into the mechanisms of the processes involved, atomic force microscopy, scanning electron microscopy and differential scanning calorimetry were also applied. As cationic polysaccharides, chitosan and cationic derivatives of hydroxypropylcellulose including hydrophobically modified samples were taken and, as their anionic counterparts, alginates, carrageenans, xanthans and fucoidans were used. Their combination allowed us to consider the influence of charge density, hydrophobicity and flexibility–stiffness of macromolecules on the association of oppositely charged polysaccharides, the formation of temperature sensitive hydrogels and some PEC morphological features.     

交联聚电解质纳米微粒的合成及研究

交联聚电解质纳米微粒的合成及研究马荣久金晓霞孙辉（吉林大学高分子研究所长春１３００２３）ＳｙｎｔｈｅｓｉｓａｎｄＳｔｕｄｉｅｓｏｆＣｒｏｓｓｌｉｎｋｅｄＰｏｌｙｅｌｅｃｔｒｏｌｙｔｅＮａｎｏｐａｒｔｉｃｌｅｓＭａＲｏｎｇｊｉｕ，ＪｉｎＸｉａｏｘｉａ...     

Polyelectrolyte solutions + multivalent salts = phase separation

Summary A queous solutions of highly charged flexible polyelectrolytes phase separate in the presence of multivalent counterions. We present a theoretical model which explains this behavior in terms of electrostatic bridging between monomers via the condensed counterions. The ?Polymer Reference Interaction Site Model? with the Debye-Hückel closure (equivalent to the ?Random Phase Approximation?) is sufficient to understand the crucial role of the valency of the counterions, to predict a demixion above a given valency and a redissolution of the precipitate in excess of added salt. The model successfully reproduces the different experimental phase diagrams of polystyrene sulfonate suspensions with different added electrolytes. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Polyelectrolyte contributions to transitions in electrostatic persistence length

The manner in which a conformational change determining the polyelectrolyte contribution to chain stiffness is incorporated into existing polyelectrolyte theory is discussed. The full Poisson-Boltzmann equation for the electrostatic potential (rather than a linear version) is incorporated into the calculation of the electrostatic persistence length. Changes in the electrostatic persistence length can then be demonstrated qualitatively to be controlled by the solution dielectric constant.     

Scaling Theory of Strongly Charged Cylindrical Polyelectrolyte Brushes

Scaling theory of charged cylindrical polyelectrolyte brushes is developed. The dependence of brush thickness on the grafting density, charge fraction, and chain length is analyzed. A full phase diagram is established. Characteristics and boundaries between different regimes of cylindrical polyelectrolyte brushes are summarized. Special attentions are paid to electrostatic interaction induced stiffening and counterion condensation effects. If the Bjerrum length of the solution is larger than the Kuhn length of the polyelectrolyte chains, counterion condensation occurs in the strongly charged polyelectrolyte brushes. On the contrary, the electrostatic interaction stretches the strongly charged grafted polyelectrolyte chains to their contour length.     

Polyelectrolyte gels in poor solvent: Elastic moduli

We study theoretically using scaling arguments the behavior of polyelectrolyte gels in poor solvents. Following the classical picture of Katchalsky, our approach is based on single-chain elasticity but it accounts for the recently proposed pearl necklace structure of polyelectrolytes in poor solvents. The elasticity both of gels at swelling equilibrium and of partially swollen gels is studied when parameters such as the ionic strength or the fraction of charged monomers are varied. Our theory could be useful to interpret recent experiments performed in Strasbourg that show that if identical gel samples are swollen to the same extent at different pH the sample with the highest charge has the lowest shear modulus. Received 7 April 2000     

Polyelectrolyte multilayer formation: Electrostatics and short-range interactions

We investigate the phenomenon of multilayer formation via layer-by-layer deposition of alternating charged polyelectrolytes. Using mean-field theory, we find that a strong short-range attraction between the two types of polymer chains is essential for the formation of multilayers. For strong enough short-range attraction, the adsorbed amount per layer increases (after an initial decrease), and finally it stabilizes in the form of a polyelectrolyte multilayer that can be repeated hundreds of times. For weak short-range attraction between any two adjacent layers, the adsorbed amount (per added layer) decays as the distance from the surface increases, until the chains stop adsorbing altogether. The dependence of the threshold value of the short-range attraction as function of the polymer charge fraction and salt concentration is calculated.     

Microstructure of Polyelectrolyte Nanoaggregates Studied by Fluorescence Probe Method

The microstructure of water soluble nanoaggregates based on polyelectrolyte complex formed by the cationic comb-type copolymer poly(acrylamide -co-[3- (methacryloyl-amino)propyl] trimethylammonium chloride)-graft- polyacrylamide [P(AM-co-MAPTAC)-g-PAM] and the anionic linear polyelectrolyte sodium polyacrylate (NaPA) was investigated using the fluorescence probe technique. The fluorescence probe were 1-anilinonaphthalene-8-sulfonic acid (ANS), pyrene (Py) and 1,10-bis(1-pyrene) decane (PD). The fluorescence properties in polyelectrolyte complex solutions, which are sensitive to either micropolarity (ANS, Py) or microviscosity (PD), were related to the quantities obtained in different pure or mixed solvents. Micropolarities were quantified utilizing the polarity common index (Reichardt) E _T(30). ANS and Py showed a variation of the micropolarity with the charge ratio of the two polymers, with the lowest polarity reached at the complex neutralization. The PD probe, by its excimer-to-monomer fluorescence intensities ratio, enabled us to evidence the effect of the composition and the comb-type copolymer grafting density on the microviscosity of the interpolyelectrolytes aggregates. It has been found that the microviscosity increased with the density of the grafting PAM chains.     

Blood Compatibility of Multilayered Polyelectrolyte Films Containing Immobilized Gold Nanoparticles

Surface material functionalization including layer‐by‐layer (LbL) polyelectrolyte films with incorporated nanoparticles is a growing field with a wide range of biomedical applications: drug reservoirs, medical devices, or tissue engineering. In parallel, gold nanoparticles (AuNPs) can be grafted by drugs and sensitive molecules using simple protocols. This study shows that AuNP behavior is modified when they are entrapped into three partner LbL films in comparison to the colloidal solution. A polycationic (polyallylamine hydrochloride (PAH)) and a polyanionic (polyacrylic acid (PAA)) polymer is used to build films based on three cycles ((PAH/AuNP/PAA)₃). To investigate the interaction with biomolecules and cells, three different films are developed changing the outer layer (either PAH or AuNP or PAA) with the same number of AuNP deposit. The best biocompatibility is observed with a polyacrylic acid outer layer. Due to the high capacity of drug grafting on gold nanoparticles, the results seem promising for the development of nanostructured biomedical devices.     

带电聚合电介质表面吸附理论研究

摘要：应用聚合电介质吸附的定标理论,根据介质和表面电介质常数的比率,考虑多化合价吸附电介质之间强相关性作用,我们提出一种表面排斥电荷的近似定标理论方法,根据这种方法把电介质表面吸附层的相图分为本质上不同的两大类。从相图可知：当表面电荷密度低（或体带相反电荷离子密度高）,这时表面和体带相反电荷离子密度几乎相同;一旦表面电荷密度足够高,就使带相反电荷的离子在表面上浓缩。据此,可确定在这个区域内,低化合价聚合电介质形成一个相关的多链状态,当化合价足够高时,由于近邻链之间的更强排斥增强,使状态转变成单链。     

带电聚合电介质表面吸附理论研究

应用聚合电介质吸附的定标理论,根据介质和表面电介质常数的比率,考虑多化合价吸附电介质之间强相关性作用,我们提出一种表面排斥电荷的近似定标理论方法,根据这种方法把电介质表面吸附层的相图分为本质上不同的两大类。从相图可知：当表面电荷密度低（或体带相反电荷离子密度高）,这时表面和体带相反电荷离子密度几乎相同;一旦表面电荷密度足够高,就使带相反电荷的离子在表面上浓缩。据此,可确定在这个区域内,低化合价聚合电介质形成一个相关的多链状态,当化合价足够高时,由于近邻链之间的更强排斥增强,使状态转变成单链。     

Molecular Simulation of Salt Ion Effect on Anionic Polyelectrolyte Chain

The effect of salt ions on anionic polyelectrolyte chain structure has been studied by molecular mechanics and molecular dynamics, and the reason for sedimentation of the anionic polyelectrolyte in the salt solution is explored. Considering sodium polyacrylate as a model compound of anionic polyelectrolytes, the conformation of polyacrylate in salt-free and CaCl₂ solution is studied. The simulation results showed that anionic polyelectrolytes in aqueous solution had an extended chain structure due to the role of strong electrostatic repulsion. After introduction of Ca²⁺ to the solution, the collapse of the anionic polyelectrolyte chain happens. By analysis of the radius of gyration and the radial distribution function, the basic reason for the collapse of anionic polyelectrolyte chains in salt solution is clarified with atomistic resolution.     

Polyelectrolyte Nanocomposite Membranes Using Imidazole-Functionalized Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane, based on DuPont Nafion/imidazole-modified nanosilica (Im-Si), for direct methanol fuel cell applications is described. Related to the interactions between the protonated imidazole groups, grafted on the surface of nanosilica, and negatively charged sulfonic acid groups of Nafion, new electrostatic interactions can be formed in the interface of Nafion and Im-Si which result in both lower methanol permeability and also higher proton conductivity. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy, thermogravimetry analysis, differential scanning calorimetry, Fourier transform infrared spectroscopy, water uptake, methanol permeability, and ion-exchange capacity, as well as proton conductivity. The Nafion/Im-Si membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, higher selectivity parameter in comparison to the neat Nafion or Nafion/silica membranes. The obtained results indicated that the Nafion/Im-Si membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

Polyelectrolyte brushes with specific charge distribution: Mean-field lattice theory

A novel polymer system containing charged diblock polymers grafted onto a planar surface, simple salt and solvent has been considered in the framework of a mean-field lattice theory. On the basis of predicted volume fraction profiles of polymer segments, free ends, block junctions, and small ions, a detailed picture of the system has emerged. It is found that the structure of the polymer brush is decisively dependent of the relative charge of the blocks. For certain conditions, bimodal profiles appear which demonstrate the simultaneous presence of two types of chain conformations, one coiled and one stretched. Received 30 August 2000     

Polyelectrolyte Nanocomposite Membranes Using Surface Modified Nanosilica for Fuel Cell Applications

The preparation and characterization of a new type of nanocomposite polyelectrolyte membrane (PEM), based on Nafion® (E. I. du Pont de Nemours and Co., Ltd., for its copolymer of tetrafluoroethylene and perfluorinated vinyl ether) and sulfonic acid (-SO₃H) or phosphotungstic acid (PWA) modified nanosilica (Si-SO₃H or Si-PWA, respectively), for direct methanol fuel cell (DMFC) applications are described. Physical characteristics of these manufactured nanocomposite membranes were investigated by scanning electron microscopy (SEM), water uptake, methanol permeability and ion exchange capacity, as well as proton conductivity. The Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes showed higher proton conductivity, lower methanol permeability and, as a consequence, a higher selectivity parameter, in comparison to the neat Nafion® or Nafion®/pristine nanosilica membranes. The obtained results indicated that both the Nafion®/Si-PWA and Nafion®/Si-SO₃H membranes could be utilized as promising polyelectrolyte membranes for direct methanol fuel cell applications.     

Polymer adsorption

We consider the adsorption of linear polymer and copolymer chains at surfaces and interfaces. We first remind de Gennes' theory for homopolymers, made of a single monomer species. Then we turn to multi block and random copolymers. In the latter case, we consider both cases when the A and B monomer species are distributed along the chain in a noncorrelated and correlated way, respectively. For each case, we discuss single chain adsorption and the saturation of the surface. Various measurable quantities are evaluated.